The exact biochemical mechanism of aldosterone action is still unclear. Our recent results show that aldosterone stimulates mitochondrial RNA polymerase activity, suggesting that mitochondrial transcription and translation may play a key role in the action of aldosterone. We will investigate whether the basis of this increased polymerase activity is a change in the physical properties of the mitochondria or an increase in enzyme proteins. Mitochondria from renal cortex and medulla and heart of sham-operated, adrenalectomized, and aldosterone-treated adrenalectomized rats will be utilized. Heart, which exhibits a different time-course of physiologic response to aldosterone, will be included to reveal whether the mechanism differs from that of kidney. The relative importance and interaction of cytoplasmic and mitochondrial protein synthesis under the influence of aldosterone will be investigated using three in vitro systems; 1) translation of cytoribosomal mRNA using a cell-free system; 2) in vitro incorporation of amino acid into mitochondria; 3) same as in 1) but in the presence of simultaneous mitochondrial protein synthesis. In separate experiments, labeled amino acid will be infused in vivo and labeled proteins of the mitochondrial matrix, inner and outer membrane fractions will be compared after electrophoretic separation. Since the mitochondrion synthesizes some subunits of certain mitochondrial enzymes, well study incorporation of labeled precursor into one of these by immunoprecipitation. The results will add to our overall knowledge of steroid hormone action and improve our understanding of mineralocorticoid-related electrolyte disorders, leading to better disease management.